Totally wrong.
Before 1935 the governments were based on right wing parties (winners of the Civil War).
Thereafter the almost only possibility was Centre - SDP coalition untill 1987. Sometimes it was added with Communists (called popular front) sometimes it was combined National Coalition Party (in fiftties) and few times either Centre or SDP were able to have minority government.
Actually there were one centre right majority government before left won significantly 1966.
It must be remembered that Centre shared ideas of universal wellfare state with Commies meaning that there were possibilty to .
Actually only since 1987 there have been rainbow coalitions (coalitions whitout Centre) since eternities.
No, I think I am right. Almost half the time, from 45-87, Finland had four-party government. The next most common option was five-party government. Four parties spent most of the time in government, including agrarians, anti-bourgeois social democrats and frequently-bourgeois Swedish speakers. The governments included communists, middle-class liberals, Swedish-speakers, and sometimes all three. These are all very unusual phenomena for European countries in the cold war (replace Swedish speakers with minority of choice), and of course the reason is the general tendency to exclude the National Coalition Party in favour of smaller parties. These parties didn't all share the same ideology just because they shared a government together - that is the fallacy of begging the question.